U.S. patent number 10,422,529 [Application Number 15/488,171] was granted by the patent office on 2019-09-24 for oxygen heat exchanger.
This patent grant is currently assigned to AGC GLASS EUROPE, L'Air Liquide Societe Anonyme Pour L'Etude Et L'Exploitation Des Procedes Georges Claude. The grantee listed for this patent is AGC GLASS EUROPE, L'Air Liquide Societe Anonyme Pour L'Etude Et L'Exploitation Des Procedes Georges Claude. Invention is credited to Eric Baudelet, Gabriel Constantin, Olivier Douxchamps, Bertrand Leroux, Bruno Symoens, Remi Tsiava.
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United States Patent |
10,422,529 |
Douxchamps , et al. |
September 24, 2019 |
Oxygen heat exchanger
Abstract
The present invention relates to a heat exchanger for the supply
of oxygen or of a gas mixture containing at least 50% oxygen, the
temperature at the outlet of the exchanger not being below
300.degree. C., it preferably being above 400.degree. C., the
oxygen or the oxygen-rich gas feeding one or more burners of a
glass melting furnace, the heat of the combustion gases being used
directly or indirectly to heat the oxygen or the oxygen-rich gas in
the exchanger, in which the exchange power is between 20 and 300
kW, preferably between 40 and 250 kW and particularly preferably
between 80 and 170 kW.
Inventors: |
Douxchamps; Olivier (Jumet,
BE), Baudelet; Eric (Jumet, BE), Leroux;
Bertrand (Paris, FR), Constantin; Gabriel (Parix,
FR), Tsiava; Remi (Paris, FR), Symoens;
Bruno (Jumet, BE) |
Applicant: |
Name |
City |
State |
Country |
Type |
AGC GLASS EUROPE
L'Air Liquide Societe Anonyme Pour L'Etude Et L'Exploitation Des
Procedes Georges Claude |
Brussels
Paris |
N/A
N/A |
BE
FR |
|
|
Assignee: |
AGC GLASS EUROPE (Brussels,
BE)
L'Air Liquide Societe Anonyme Pour L'Etude Et L'Exploitation Des
Procedes Georges Claude (Paris, FR)
|
Family
ID: |
38577527 |
Appl.
No.: |
15/488,171 |
Filed: |
April 14, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170217813 A1 |
Aug 3, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13730727 |
Dec 28, 2012 |
9803860 |
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12599580 |
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PCT/EP2008/055615 |
May 7, 2008 |
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Foreign Application Priority Data
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May 10, 2007 [EP] |
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07107942 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28F
19/00 (20130101); F28F 21/08 (20130101); F23L
15/04 (20130101); C03B 5/2353 (20130101); F28D
7/16 (20130101); Y02E 20/34 (20130101); Y02E
20/344 (20130101); Y02P 40/50 (20151101); Y02P
40/55 (20151101) |
Current International
Class: |
C03B
11/08 (20060101); F23L 15/04 (20060101); F28F
21/08 (20060101); C03B 5/235 (20060101); F28F
19/00 (20060101); F28D 7/16 (20060101) |
Field of
Search: |
;65/374.1
;165/64,108,168,169,170,171,172,173,174,175,176,177,179 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 068 057 |
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Jan 1983 |
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EP |
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0 650 934 |
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May 1995 |
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EP |
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0 801 035 |
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Oct 1997 |
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EP |
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0 872 690 |
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Oct 1998 |
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EP |
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1 094 273 |
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Apr 2001 |
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EP |
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1 136 451 |
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Sep 2001 |
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EP |
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2 863 690 |
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Jun 2005 |
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FR |
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2000 254484 |
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Sep 2000 |
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JP |
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Other References
Lievre, K., et al., "Recent developments in oxy-fuel firing for
glass melters," Glass Industry, Glass Industry Magazine, vol. 82,
No. 3, pp. 25-31, (Mar. 1, 2001). cited by applicant .
International Search Report dated Sep. 10, 2009 in PCT/EP09/056843
filed Jun. 4, 2009. cited by applicant .
Ruiz, R. et al., "Oxy-Fuel Furnace Design Considerations", Glass
Industry, vol. 76, No. 6, pp. 10, 13-14, 19-20 and 23, XP
000506664, ISSN: 0017-1026, (May 1, 1995). cited by applicant .
Wu, K. T. et al., "Design Modeling of Glass Furnace Oxy-Fuel
Conversion Using Three-Dimensional Combustion Models", Ceramic
Engineering and Science Proceedings, vol. 17, No. 2, pp. 132-141,
XP000622573, ISSN: 0196-6219, (Jan. 1, 1996). cited by applicant
.
International Search Report dated Jun. 10, 2009 in PCT/EP09/053506
filed Mar. 25, 2009. cited by applicant .
International Search Report dated Jun. 4, 2009 in PCT/EP09/53500
filed Mar. 25, 2009. cited by applicant .
International Search Report dated Jun. 5, 2009, in PCT/EP09/053518
filed Mar. 25, 2009. cited by applicant .
International Search Report dated Jun. 5, 2009 in PCT/EP09/53522
filed Mar. 25, 2009. cited by applicant .
Kobayashi, Hisashi et al., "Advances in Oxy-Fuel Fired Glass
Melting Technology", Proceedings of International Congress on
Glass, pp. 1-17, XP002496632, (2004). cited by applicant .
Joseph R. Davis, "Elevated-Temperature Corrosion Properties of
Superalloys" ASM Internationi Handbook Committee ASM International,
1997, retrieved from:
http//books.google.com/books?id=GEHA8-bix0oC&pg=PA2888&,dq=Elevated-Tempe-
rature+Corrosion+Properties+of+Superallows%22+ASM+International+Handbook+C-
ommittee+ASM+International+1997. cited by applicant .
Material Data Sheets for Inconel, Hydra, retrieved from:
http//www.witzenmann.de/download/Manual%20of%20metal%20bellows_0441e%20S%-
20174-199_2_04_10_20web.pdf. cited by applicant .
Bott, T. Reg. to Foul or Not to Foul That is the Question, Chemical
Engineering Progress Magazine, Nov. 2001, pp. 30-37, Retrieved on
Oct. 28, 2014, Retrieved from:
www.clarkson.edu/-wilcox/Design/hxfoul.pdf. cited by
applicant.
|
Primary Examiner: Wilson; Michael H.
Assistant Examiner: Krinker; Yana B
Attorney, Agent or Firm: Oblon, McClelland, Maier &
Neustadt, L.L.P.
Parent Case Text
This application is a Continuation of U.S. Ser. No. 13/730,727
filed Dec. 28, 2012, allowed, which is a divisional of U.S. Ser.
No. 12/599,580, filed Jun. 25, 2010, abandoned, which is a 371
application of PCT/EP08/55615 filed May 7, 2008, and claims the
benefit of EP07107942.0 filed May 5, 2010. The contents of each of
these applications are incorporated herein by reference in their
enterity.
Claims
What is claimed:
1. A process for supplying gas to one or more burners of a glass
melting furnace, comprising: supplying combustion gases from the
glass melting furnace to a first heat exchanger; effecting a first
heat exchange using the combustion gases to heat an intermediate
heat transfer gas to form a heated heat transfer gas in the first
heat exchanger; supplying oxygen gas comprising oxygen or a gaseous
mixture comprising at least 50% oxygen to a second heat exchanger
having an outlet; effecting a second heat exchange by heating the
oxygen gas in the second heat exchanger with the heated heat
transfer gas to a temperature at the outlet of the exchanger of not
less than 300.degree. C. to form heated oxygen gas; supplying the
heated oxygen gas from the outlet to one or more burners of the
glass melting furnace; exchanging a power per unit area in contact
with the oxygen or gas rich in oxygen in the second heat exchanger
in the range of between 5 and 15 W/m.sup.2; circulating the oxygen
gas in the tubes carrying the oxygen gas at a rate that does not
exceed 120 m/s at any point in the second heat exchanger;
maintaining a pressure of the oxygen gas in the second heat
exchanger below 3 bar, wherein both the first and second heat
exchanges are indirect heat exchanges.
2. The process according to claim 1, wherein each second heat
exchanger supplies heated oxygen gas to at most three burners of
the furnace.
3. The process according to claim 1, further comprising:
circulating the oxygen gas in tubes in the second heat exchanger,
and contacting internal walls of the second heat exchanger with the
heated heat transfer gas, wherein the second heat exchanger has a
tubular configuration.
4. The process according to claim 3, in which the tubes in which
the oxygen gas circulates are substantially straight and walls of
the tubes have a thickness that is not more than 3 mm.
5. The process according to claim 3, in which a chamber enclosing
the tubes is formed from several elements joined by flanges,
wherein tightness is assured at these flanges by composite seals, a
sealing element of which is made of material that is inert with
respect to oxygen.
6. The process according to claim 5, in which the sealing element
is a ring comprising compressible mineral material.
7. The process according to claim 3, further comprising: placing an
oxygen detector in contact with the heat transfer gas, and
connecting the oxygen detector to an alarm when an oxygen content
is more than 1% higher than that of the heat transfer gas.
8. The process according to claim 1, wherein a material of surfaces
in contact with the oxygen gas in the second heat exchanger is made
from a metal alloy of which a sample exposed to the oxygen gas does
not exhibit a weight gain of more than 0.1 mg/cm.sup.2 after 1000
cycles of exposure, wherein each cycle includes increasing a
temperature of the oxygen gas to a value equal to or higher than
400.degree. C., maintaining this phase temperature for one hour and
returning to ambient temperature.
9. The process according to claim 8, in which the alloy complies
with the condition of a weight gain of less than 0.1 mg/cm.sup.2 of
exposed surface when a phase temperature is at least 500.degree. C.
in oxidising atmosphere.
10. The process according to claim 8, in which the alloy in contact
with the oxygen gas resists a spontaneous combustion test according
to standard ASTM G 124 at least up to a pressure of 3 bar.
11. The process according to claim 8, in which the alloy in contact
with the oxygen gas is a ferritic steel alloy containing a
percentage by weight of Cr of 12 to 30% and an Al content of 1 to
8%.
12. The process according to claim 8, in which the alloy in contact
with the oxygen gas, for an oxygen temperature not exceeding
500.degree. C., is an alloy containing a percentage by weight of
chromium in a range of between 10 and 20% by weight.
13. The process according to claim 8, in which the alloy has a Ni
content higher than 25% and a Cr content from 10 to 30%.
14. The process according to claim 13, further comprising bringing
elements in the heat exchanger in contact with the oxygen gas to a
temperature in a range of between 300.degree. and 900.degree.
C.
15. The process according to claim 8, in which the alloy complies
with the condition of a weight gain of less than 0.1 mg/cm.sup.2 of
exposed surface when the phase temperature reaches at least
600.degree. C. and an oxidising atmosphere exceeds 80% oxygen.
16. The process according to claim 8, in which the alloy complies
with the condition of a weight gain of less than 0.1 mg/cm.sup.2 of
exposed surface when the phase temperature is at least 650.degree.
C. in oxidising atmosphere.
17. The process according to claim 1, wherein a power exchanged in
the second heat exchanger to heat the oxygen gas is in a range of
between 40 and 250 kW.
18. The process according to claim 1, wherein a power exchanged in
the second heat exchanger to heat the oxygen gas is in a range of
between 80 and 170 kW.
19. The process according to claim 1, further comprising
maintaining a pressure of the oxygen gas in the second heat
exchanger below 2 bar.
20. The process according to claim 1, where the heated oxygen gas
at the outlet is at a temperature of not less than 400.degree.
C.
21. The process according to claim 1, further comprising each
burner consuming heated oxygen at a rate of between 200 to 1200
Nm.sup.3 per hour.
22. The process according to claim 1, further comprising heating
the heat transfer gas to between 450.degree. C. and 1000.degree.
C.
23. The process according to claim 1, further comprising generating
in each burner from 1 to 6 MW in the furnace.
24. A process for supplying gas to one or more burners of a glass
melting furnace, comprising: supplying combustion gases from the
glass melting furnace to a first heat exchanger; effecting a first
heat exchange using the combustion gases to heat an intermediate
heat transfer gas to form a heated heat transfer gas in the first
heat exchanger; supplying oxygen gas comprising oxygen or a gaseous
mixture comprising at least 50% oxygen to a second heat exchanger
having an outlet; effecting a second heat exchange by heating the
oxygen gas in the second heat exchanger with the heated heat
transfer gas to a temperature at the outlet of the exchanger of not
less than 300.degree. C. to form heated oxygen gas; supplying the
heated oxygen gas from the outlet to one or more burners of the
glass melting furnace; circulating the oxygen gas in tubes carrying
the oxygen gas at a rate that does not exceed 120 m/s at any point
in the second heat exchanger; maintaining a pressure of the oxygen
gas in the second heat exchanger below 3 bar, wherein both the
first and second heat exchanges are indirect heat exchanges, and
wherein surfaces of the second heat exchanger contacting the oxygen
gas are polished to have a roughness that does not exceed 6
.mu.m.
25. The process according to claim 24, in which the intermediate
heat transfer gas is air, nitrogen, CO.sub.2 or steam.
26. The process according to claim 24, in which the intermediate
heat transfer gas is formed from combustion gases diluted by means
of at least one of the gases: air, nitrogen, CO.sub.2 and
steam.
27. The process according to claim 24, further comprising: heating
the intermediate heat transfer gas in a recuperator, which has been
heated by the combustion gases beforehand.
28. The process according to claim 24, further comprising
maintaining a pressure of the oxygen gas in the second heat
exchanger below 2 bar.
Description
The present invention relates to heat exchangers intended to heat
oxygen or a gas rich in oxygen for the purpose of supplying the
burners of glass melting furnaces.
Glass melting furnaces, including those with the highest production
capacity, i.e. the furnaces supplying the "floats" producing flat
glass, are mostly equipped with burners that operate with fossil
fuels and air. The choice of this type of energy is driven by
economic reasons, considering the importance of energy consumption.
As an indication, usual melting furnaces producing between 600 and
900 tonnes of glass per day require an available power in the order
of 50 to 80 megawatts.
The use of these furnaces not only results in a significant energy
consumption but also in the discharge of large volumes of
combustion gas. For these reasons, efforts are constantly being
made to reduce energy costs and the costs for processing these
combustion gases as far as possible.
The currently most widespread practice using air as combustive does
not lead to the lowest possible consumption of energy, since
nitrogen in the air is also brought to high temperature and
consequently absorbs a significant portion of the energy released
by combustion, and part of this energy is lost even if the fumes
undergo a recovery operation. Moreover, as is well known, the
presence of nitrogen leads to the formation of the oxides
responsible for so-called acid rain.
For these reasons solutions that use not only air but oxygen or
gases rich in oxygen have been developed and are already being used
in some applications.
However, the systematic use of oxygen encounters practical and
economic difficulties. The costs of the oxygen required is the
first of the economic difficulties. The energy balance gain must
more than compensate the additional cost of the oxygen. The
investments in specific material are also of importance in the
economics of the system, whether it concerns the burners, supplies
or, as studied below, the exchangers. In practice, the continuous
supply of oxygen to these large-scale assemblies requires
production installations on site or supply via gas pipeline, either
of which incurring substantial investments.
Considering the burdens imposed by the use of oxygen or gas rich in
oxygen in these glass melting furnaces to reach a satisfactory
economic balance, it is necessary to optimise the choices made.
It appears that the best energy balance requires that burners are
run using oxygen and preheated fuels, with at least part of the
combustion gases being used for this preheating operation.
While the use of hot oxygen is highly recommendable, putting this
into practice requires new solutions both for the configuration of
these installations and for their operation. Thus, traditionally,
the air used for the burners is preheated in regenerators. These
are towers lined with refractory materials, into which the
combustion gases are passed to heat the refractory materials in a
first phase and into which the air used in the combustion is passed
for reheating in a second phase. The alternation of these phases
results in a highly specific furnace structure. The burners are
thus on either side of the melting bath as are the regenerators
associated with them that are generally located on the opposite
side to the active burners.
It is not possible to use regenerators to reheat the oxygen.
Generators are commonly the receptacle for deposits of particles
carried by the combustion gases, even if these have been subjected
to a dust removal operation beforehand. Contact of the hot oxygen
with these deposits is not without risk. Moreover, it is difficult
to guarantee a perfect seal of these regenerators. The passage of
air and possible leaks are not dangerous, but this is not the case
with respect to oxygen.
The use of heat exchangers also raises sensitive problems of
resistance of the materials to contact with hot oxygen. In view of
these difficulties and taking into account economic constraints,
the use of hot oxygen has not found significant application in
large installations such as those for large-capacity glass melting
furnaces.
Therefore, the aim of the invention is to propose solutions that
encourage the use of oxygen or hot gases rich in oxygen in the
burners of glass melting furnaces, and in particular in
large-capacity furnaces. The invention also proposes to provide
solutions that make this use sufficiently safe despite the special
technical requirements associated with the use of oxygen at high
temperature.
Since the risks associated with the use of oxygen occur in the
installation from the instant the temperature increases, the
inventors have directed all their attention to the devices in
contact with this hot oxygen and in particular to the exchangers,
which contain significant quantities of this oxygen and whose
construction must allow operation without failure for very long
periods, because of the difficulties raised by any intervention for
maintenance or repairs.
The above considerations relating to the use of hot oxygen also
apply to gas mixtures, in particular with air, in which the oxygen
content is sufficiently high. In practice, for these gas mixtures
to remain advantageous in use, their oxygen content should not be
less than 50%. This condition applies to the invention presented
below. The invention preferably applies to gas mixtures with an
oxygen content of at least 80%.
According to the invention, the heating of oxygen or gas rich in
oxygen for supplying the burners of the furnace is conducted in
exchangers, in which the exchange power is deliberately reduced
without minimising the temperature to which the oxygen or gas rich
in oxygen is brought.
To sufficiently benefit the advantage for the energy balance, the
temperature of the oxygen or gas rich in oxygen at the outlet of
the exchanger is not less than 300.degree. C. and preferably not
less than 400.degree. C., and the power exchanged in the exchanger
to bring the oxygen to these temperatures according to the
invention lies between 20 and 300 kW, preferably between 40 and 250
kW and particularly preferred between 80 and 170 kW.
Power levels lower than those recommended are not economical and
impair the competitiveness of the system. Higher power levels are
likely to lead to difficulties with respect to convenience and,
above all, the safety of use of these exchangers.
In traditional glass melting furnaces, at least those of large
capacity, the burners used generate significant power levels in the
order of 1 to 6 MW resulting in an oxygen consumption in the order
of 200 to 1200 Nm.sup.3 of oxygen per hour.
In view of the power, and therefore consumption of each of the
burners typically used in these furnaces, the exchangers according
to the invention are only associated with a small number of
burners. Each exchanger advantageously only supplies at most three
separate burners simultaneously with hot oxygen or gas rich in
oxygen, wherein each burner can have several injection nozzles,
depending on the specifications, such as those presented, for
example, in EP 1 194 719.
This arrangement that leads to an increase in numbers of exchangers
assures an improved safety of the installation by limiting the
dimensions and capacity of each exchanger, but also significantly
limiting the length of the ducts downstream of the exchangers that
lead to the burners.
In order to remain advantageous from the economic viewpoint, the
use of exchangers according to the invention must preferably result
in limited dimensions, which involves a quite specific mode of
operation and in particular allows the required power to be
generated while also keeping the exchange area as small as
possible.
According to the invention the exchangers for heating the oxygen or
gas rich in oxygen advantageously have a power per unit area of
contact of the oxygen with the exchange walls in the range of
between 5 and 15 kW/m.sup.2, and preferably between 7 and 12
kW/m.sup.2. The area in question is that of the wall separating the
oxygen or gas rich in oxygen from the heat transfer gas.
The construction of the exchangers used according to the invention
must provide as simple a structure as possible to prevent risks of
erosion and leakage as a result of the aggressiveness of hot oxygen
towards materials used. For convenience, the exchangers according
to the invention are preferably tubular, wherein the oxygen or gas
rich in oxygen circulates in a bank of tubes with the heat transfer
gas circulating on the outside of these.
To guarantee the power levels indicated above, and in particular
the power levels per unit area, while keeping these areas as small
as possible, it is necessary to optimise the heat exchange
conditions.
A first way to benefit this exchange consists of increasing the
circulation rate of the gases and in particular the oxygen or gas
rich in oxygen. However, from this viewpoint it is necessary once
again to take into account that the increase in circulation rate is
a risk factor. The risk is all the more significant as hot oxygen
is likely to entrain particles that can react with the oxygen
and/or whose impact on the walls promotes rapid erosion in addition
to that resulting from the friction of the oxygen itself.
In order to take this risk into account, according to the invention
the dimensions of the elements of the exchanger are advantageously
defined so that in order to obtain the necessary power level, the
circulation rate of the oxygen or gas rich in oxygen is not higher
than 120 m/s at any point in the exchanger, and preferably is not
higher than 100 m/s.
Besides the temperature and circulation rate of the oxygen, it is
known that the risk of ignition in particular is also dependent on
pressure. If an increase in pressure leads to a reduction in volume
and therefore in the circulation rate in the exchanger, a
modification of this type is also dependent on the operating
characteristics of the burners that can be used in the furnaces in
question. In practice, it is a very delicate matter to proceed
towards a pressure reduction of the hot oxygen after passage into
the exchanger and before it reaches the burner. Because of its
nature, it does not reliably allow passage into pressure reducing
means, and even the use of diaphragms to conduct this operation is
not recommended, both as a result of its aggressiveness. A high
pressure at the level of the injection nozzle or nozzles of the
burner to maintain an adequate delivery rate would result in the
opening of this or these nozzles to be reduced. This is not
desirable because of the risk of fouling and/or wear of these
nozzles, which would quickly lead to defective operation.
To take these factors into account, the exchangers according to the
invention are also dimensioned such that for the power levels
sought, the pressure of the oxygen or gas rich in oxygen in the
exchanger does not exceed 3 bar, preferably not 2 bar and
particularly preferred 1.5 bar.
Studies of the operating specifications of these installations show
that the best energy balances require that the fuels and oxygen
carriers are brought to an elevated temperature. The temperatures
to which the gases must be brought are as elevated as the
installations used permit.
While it is advantageous from the point of view of efficiency of
the burners to bring the oxygen to the most elevated temperatures,
these are limited in practice by the temperature of the heat
transfer fluid taking part in the exchange as well as by the need
to limit the risk of corrosion or even ignition of the materials in
contact with the oxygen.
The energy supply to heat the oxygen or gas rich in oxygen comes
from the combustion gases either directly by circulation in the
exchanger or preferably indirectly by means of a fluid that has
itself been reheated beforehand by an exchange with the combustion
gases.
This latter manner of proceeding provides additional safety in the
case of deficient tightness in the circulation of the oxygen. Rapid
fouling of the exchanger can also be prevented as a result, since
the combustion gases entrain a significant quantity of dust with
them that it is difficult to separate completely.
The intermediate gas, should such a double exchange occur, is
advantageously inert with respect to oxygen. This is preferably
air, nitrogen, CO.sub.2, steam or a mixture of these gases.
If necessary, the intermediate gas can be formed from a mixture of
the inert gases indicated above and a portion of the combustion
gases that have undergone dust separation beforehand.
The temperature of the fumes can increase to 1550.degree. C. and
most frequently lies between 1250.degree. and 1450.degree. C. and
is higher than the temperatures, to which oxygen can be brought
without too severely degrading the material of the walls with which
it comes into contact.
In the case of an installation comprising a double exchange, in
which the heat transfer fluid is formed from air, the temperature
of this latter after being reheated by the combustion gases is
preferably in the range of between 450.degree. and 1000.degree. C.
and particularly preferred between 600.degree. and 800.degree.
C.
As clearly evident from the studies of the inventors, which form
the subject of the tests outlined below, the corrosion of the
materials forming the heat exchanger increases rapidly with the
temperature such that it is preferable to keep the oxygen in
conditions that ensure against excessive corrosion.
In practice, the temperature of the hot oxygen or gases rich in
oxygen as results from the heat exchanges remains within the limits
where the choice of materials made according to the invention can
prevent excessive corrosion of the installation. This temperature
does not ordinarily exceed 900.degree. C. and preferably is not
higher than 700.degree. C.
The materials forming the exchanger, and primarily those in contact
with the hot oxygen, must be selected in order to assure a good
resistance to oxidation by the gases and in particular the oxygen
in these temperature conditions.
The steels most commonly used are not sufficiently resistant.
Consequently, it is necessary to make a particularly careful
selection in order to guarantee long service life of the
installation.
The selection of materials not only includes consideration of
resistance to the highest temperatures reached in these
installations, but also a good resistance to temperatures that are
lower, but are also known to cause a change in state of the
material which will make it particularly sensitive to possible
degradations. During an increase in temperature, some steels in
particular pass through transition temperature zones that will
cause embrittlement of the metal.
There are several aspects to the desired resistance. It is not just
a matter of preventing vigorous oxidation of the materials in the
form of their combustion, it is also a matter of preventing any
modification of the surface in contact with the oxygen, which can
eventually not only lead to perforation of the walls but much more,
it is advisable in some cases to prevent the detachment of
particles that are likely to disturb the subsequent reaction and/or
pollute the products prepared by means of the combustion maintained
with this hot oxygen.
In all cases, the installation in which the gases rich in oxygen
circulate must be made from materials that naturally provide the
desired resistance to these gases that are particularly aggressive
by nature, and all the more so when the temperature, circulation
rate and pressure are more elevated.
So that the exchanger used to heat the oxygen or gas rich in oxygen
can meet the conditions of use outlined above in a lasting manner,
the inventors have established characteristics that the materials
forming them must meet.
In general, according to the invention the exchanger that must
receive gases containing at least 50% oxygen at a temperature not
less than 300.degree. C. is made, at least in the case of the walls
directly in contact with these gases, of a metal alloy that
complies with the following test protocol.
A sample of metal alloy according to the invention placed in an
atmosphere corresponding to the gas rich in oxygen that has to
circulate in the installation and at the most elevated temperature
encountered in the installation does not exhibit a weight gain of
more than 0.1 mg/cm.sup.2 of surface exposed after 1000 cycles each
including maintaining the anticipated maximum temperature for 1
hour, each phase at this temperature being followed by a return to
ambient temperature.
The choice of repeated passage through the elevated temperatures
followed by a return to ambient temperature is particularly
exacting. The structures of the metal can encounter repeated phase
modifications in particular that cause the highest stresses, and
therefore increased risks of degradation. In practice, this test is
such that it subjects the metal to more significant trials than
those withstood during use of the industrial exchanger itself,
which is intended to operate continuously, this operation only
being interrupted occasionally for maintenance operations at the
longest possible time intervals.
As indicated above, since the burners of the glass furnaces are
preferably supplied with a gas with an oxygen content that is
preferably higher than 80% and can reach 100%, the test indicated
above must advantageously be passed for these oxygen contents.
To best assure a long service life of the exchanger, the chosen
metal alloy goes through the same test, but here the control
temperature is at least 500.degree. C., and to meet the envisaged
extreme conditions, the alloy went through the test in which the
most elevated test temperature is at least 600.degree. C., and can
pass this test even at temperatures of 800.degree. C.
In addition, in a test for spontaneous combustion in an oxidising
atmosphere in accordance with standard ASTM G-124, the alloys most
suitable for forming the exchanger according to the invention
resist this combustion at least up to pressures of 3 bar and
preferably at least up to pressures of 10 bar.
Those alloys advantageously used and having a positive response to
the corrosion test when used in temperature ranges above
550.degree. C. include ferritic type non-oxidising alloys, in which
the Cr content is in the range of between 12 and 30% by weight and
which simultaneously contain Al at the rate of 1 to 8%.
Ferritic alloys are subject to embrittlement when in temperature
ranges between 400.degree. and 500.degree. C. For these reasons,
the use of these alloys must take into account the considered
factors and conditions, in particular temperature conditions,
prevailing in the exchanger.
The parts of the exchanger exposed to hot oxygen can also be made
from alloys rich in Ni and Cr having Ni contents higher than 25% by
weight and simultaneously containing 10 to 30% Cr. The Ni content
can rise to 75% or more.
These alloys differ from one another in particular in their
mechanical properties. Moreover, their selection must possibly take
into account any limitations specific to the envisaged use. While
alloys with a high Ni content work well in flat glass production
installations, it is important to take into consideration the risk
posed by the presence of Ni, as any entrainment of particles by the
Ni must be carefully avoided because of the risk of nickel sulphide
forming in the glass sheets that generates fractures.
These alloys have a good resistance to corrosion at elevated
temperature due to the formation of a protective film of chromium
or aluminium oxide. The chromium content of the alloy must be
sufficiently high in order to prevent the formation of nickel oxide
nodules that increase rapidly and, if entrained, would be capable
of forming nickel sulphide in the glass sheets that generates
fractures.
With oxygen temperature ranges higher than 500.degree. C., and in
particular between 500.degree. and 700.degree. C., it is preferable
to use alloys in which the chromium content is only 10 to 20%,
particularly preferred between to and 16%. Alloys rich in Ni that
meet the specified requirements are in particular those usually
referred to by the names Inconel 600H, 600L, Incoloy 800H.
With oxygen temperature ranges higher than 100.degree. C., and in
particular between 100.degree. and 600.degree. C., it is preferable
to use alloys in which the chromium content is higher than 16%,
particularly preferred between 20 and 30%. Alloys rich in Ni that
meet the specified requirements are in particular those usually
referred to by the names Inconel 600H, 600L, 601, 617, 625, Incoloy
800H and 800HT.
It is also possible to use an alloy such as stainless steels 316L
and 310, which are easy to work, but whose service life is less
assured.
While the circulation rate of the highly oxidising gases at
elevated temperature is a risk factor with respect to erosion, this
can be increased by particles carried by these gases. Initially,
the gases are substantially free of solid particles, but these can
come from the installation itself. The walls of the ducts and the
heat exchangers exposed to corrosion by these gases can in fact
release particles, which as they impact the elements downstream
also generate erosion and to a much higher degree, as the flow rate
of the gases increases.
The surface condition of the walls of the exchanger can affect the
resistance to corrosion. The more pronounced the surface
irregularities are, the more the alloy is corroded with otherwise
identical conditions. For this reason, the surfaces of the walls of
the exchanger according to the invention that come into contact
with the gases rich in oxygen are polished and have a roughness of
not more than 6 micrometers (.mu.). The roughness is preferably
less than 4.mu. and most advantageously is at most equal to
2.mu..
Examples of practical details of the invention are given in the
description below with reference to the set of drawings:
FIG. 1 is a schematic sectional illustration of a gas exchanger
usable according to the invention to reheat oxygen or gas rich in
oxygen;
FIG. 2 is a partially enlarged view of the end of the exchanger
shown in FIG. 1;
FIG. 3 shows a detail of part-section A taken from FIG. 2.
The general structure of the exchanger is the conventional type for
gas exchangers. It comprises a chamber 1 enclosing a bank of tubes
2. The tubes are secured inside the chamber by plates 3, 4.
The plates form a sealed wall delimiting the zone of the chamber 1,
in which the heat transfer gas circulates.
The chamber is closed at its ends by two covers 5, 6. These covers
are tightly secured to the chamber by means of flanges 7, 8, 9, 10
and seals. These flanges can be removed to give access to the ends
of the tubes 2, where necessary.
To obtain the best possible exchange, the circulation of the heat
transfer gas and the oxygen or gas rich in oxygen is advantageously
conducted in reverse flow. The hot heat transfer gas passes into
the chamber through conduit 11 and exits through conduit 12 after
having passed through the circuit created by the baffles 13, 14, 15
inside the chamber.
The oxygen or gas rich in oxygen circulates in the tubes 2 along a
substantially rectilinear course. It passes cold through end 16 and
exits hot at end 17 to be conducted to the burners.
The exchange is all the more effective when circulation rates are
higher. Nevertheless, the flow rate and pressure of the oxygen must
be held within the limits that assure the operating safety of the
device. The circulation of the oxygen must be prevented from
resulting in excessive corrosion of the walls which it comes into
contact with. It should also be ensured that the oxygen does not
strike the walls. The use of rectilinear tubes thus restricts
erosion.
The arrangement of the ends of the tubes 2 is shown in detail in
FIG. 2.
To prevent turbulence at the ends of the tubes, with the risks of
increased erosion at the point where these tubes connect to the
plate 4 most frequently by suitable welds, these tubes terminate
with a widened section. This arrangement facilitates the flow of
oxygen and its expansion and subsequently some deceleration. This
widening is in the shape of a truncated cone in the figure with an
angle of opening .alpha..
For the same reason, the covers, and above all cover 6 arranged at
the oxygen outlet, are located at a distance from the ends of the
tubes 2. In this way, the flow rate of the oxygen along the walls
of the cover is substantially reduced in relation to that at the
outlet of the tubes.
The general shape of this cover 6 is also chosen so that the
advance of the hot oxygen encounters the wall of the cover at a low
incidence, thus minimising impact. For example, the wall of the
cover is at an angle of about 20 to 30 degrees relative to the
direction of the tubes 2. The profile of the cover decreases
progressively up to the connection with the outlet duct.
It is also advantageous to ensure that there are no sharp angles or
welds in this section.
The dimensioning of the tubes and their distribution are such that
the flow rate and pressure conditions indicated above are met by
the delivery rates implemented.
Since the exchanger must operate continuously over very long
periods, it may eventuate that a tube no longer has the necessary
tightness in spite of precautions taken to prevent wear of the
elements of the exchanger. The assembly of the exchanger is such
that the defective tube can be blocked at these two ends. The
operation requires that the covers be removed. After the defective
tube has been taken out of service, the exchanger is once again
usable with an efficiency that is little changed in proportion to
the remaining active tubes.
The tightness at the level of the flanges of the covers 9, 10 of
the exchanger or at the connection of these covers with the oxygen
intake or outlet ducts is advantageously obtained by means of a
metal annular seal 18 lined with a material 19, 20 resistant to
oxygen. The material in question is mica or a compressible mineral
material, for example. Seals of this type are produced in
particular by Garlock under the brand name "Vitaflex".
In order to determine the alloys that comply with the
implementation conditions according to the invention, the inventors
have conducted tests that are discussed in the following
description.
For these tests, the samples are formed from 2 mm thick plates of
metal alloy measuring 20.times.20 mm.
The condition of the surface of the samples indicates its clear
importance with respect to sensitivity to oxidation. For this
reason, one face of each of these plates is polished with an
abrasive sheet of SiC to grain size 1200. The other face is left in
its original state as produced by the industrial rolling
process.
The composition by weight of the samples of alloys tested is
specified in the following table:
TABLE-US-00001 Alloy Fe C Si Cr Al Ni Mn Others I 6-10 0.15 0.5
14-17 72 0.5 Cu II comp <0.04 <1.0 19-23 0.15-0.6 30-34
<1.5 0.15-0.6 Cu III comp 20 5.5 0.5 Ti 0.5 Y.sub.2O.sub.3 IV
comp 1-2 24 1-2 V comp 22 5 VI comp 0.15 23-27 <1.0 1.5
The measurement of the oxidation of the samples is evaluated by the
increase in their mass after testing over a thousand cycles. The
results for different phase temperatures are indicated in the
following table:
TABLE-US-00002 Alloy I V Temperature 550 650 800 550 650 800
mg/cm.sup.2 0.013 0.06 0.347 0.004 0.02 0.099 Alloy III IV
Temperature 550 650 800 550 650 800 mg/cm.sup.2 0.097 0.10 0.232
0.026 0.05 0.103
It is evident from these tests that the oxidation is more
significant when the phase temperature is higher. At 550.degree. C.
the increase in all cases remains well below 0.1 mg/cm.sup.2. At
650.degree. C. only one sample reaches this value. At 800.degree.
C. the most resistant samples are those of alloys IV and V.
The metallographic observation of the samples shows a much lower
tendency towards oxidation in the polished face of the samples.
The above measurements at the same time include the oxidation of
the two faces of the sample. Since only one face is polished, the
oxidation measurement obtained is thus higher than that which would
be observed in practice when the surface in contact with the oxygen
is polished.
Since the tests were conducted in a static manner, in other words
without circulation of the atmosphere in relation to the sample, no
"scale" appeared to have detached from the surface.
Analysis of the modification of the compositions at the surface,
and in particular the decrease in Cr content, is a means of
evaluating the risk of detachable particles forming. The presence
of Cr with a content of not less than 7% guarantees the formation
of a protective layer that prevents the formation of scale.
The measurements outlined in the following table show that the Cr
content remains well above these values. After testing over 1000
cycles, in which the phases are at the maximum temperatures
indicated, the analysis in percentage by weight of the samples at
the surface (S) and at the core (C) of the product leads to the
results indicated in the following table:
TABLE-US-00003 Alloy .degree. C. Cr Si Al Fe Ti Mn Ni III 800 S
17.9 6.3 72.7 0.4 0.1 C 19 5.6 73.6 0.5 0.0 IV 800 S 22.1 0.8 2.1
72.1 0.0 0.4 C 23.1 0.8 1.9 71.9 0.3 0.4 IV 550 S 18.6 0.7 1.8 75.8
0.0 0.3 C 23.7 0.8 1.9 71.8 0.0 0.4 V 800 S 20 0.0 6.3 71.0 0.0 0.1
C 21.5 0.0 5.6 71.4 0.0 0.1 I 800 S 10.9 0.2 8.8 79.5 C 15.7 0.2
7.9 75.5 I 550 S 15.6 0.1 7.5 75 C 15.9 0.1 8.0 74.2
Considering the nature of the anticipated atmosphere, the use of
materials must comply with strict safety conditions. The risk of
combustion of the material brought to elevated temperature in the
presence of pure oxygen is thus evaluated in accordance with the
protocol of standard ASTM G 124.
In these tests, specimens of material placed in an atmosphere of
oxygen under pressure are subjected to a combustion test. The
results of these tests show that at 550.degree. C. and at a
pressure of 3 bar, combustion does not occur in any of the
samples.
When the pressure or temperature is increased, the tendency towards
combustion increases. Alloy III was found to be the most sensitive
to this test.
In general, at the temperatures envisaged above, the pressure must
not exceed 10 bar, whatever alloy is selected. On this condition,
the test in accordance with the standard shows that use in supply
installations for gas rich in oxygen does not cause any risk of
combustion.
On the basis of the results of these tests for resistance to hot
oxygen, a particularly interesting point appears to be that in an
exchanger according to the invention the thickness of the walls can
be relatively less thick than one would assume from the prior art.
Longevity simulations based on these results lead to walls for the
tubes of the exchangers according to the invention that have a
thickness that can be no more than 3 mm. This thickness can even be
equal to or less than 2.5 mm.
The relatively low thickness of the walls of the tubes of the
exchanger benefits the heat transfer and therefore increases the
available power for the same exchange area.
As an exemplary embodiment, an exchanger according to the invention
is configured in the following manner. It is formed by a bank of 40
tubes of Inconel 600. The outside diameter of the tubes is 17.2 mm
and the thickness of the wall is 2.3 mm. The tubes have a length of
4000 mm.
The exchange area in contact with the oxygen is therefore 8.4
m.sup.2.
Coming from a first exchanger, the heat transfer gas (air with dust
extracted) enters the exchanger at a temperature of 650.degree. C.
The delivery rate of the heat transfer gas is set at 750
Nm.sup.3/h. The delivery rate of oxygen is 400 Nm.sup.3/h. As it
enters at ambient temperature the oxygen is heated to 550.degree.
C.
The flow rate of the oxygen in the ducts is 67 m/s and the load
loss in the exchanger is less than 0.15 bar. A safety system
comprising a pressure controller maintains the pressure in the
exchanger at less than 1 bar.
The nominal power of the exchanger is 84 kW and per unit area is
set at 9.7 kW/m.sup.2.
The exchanger supplies a burner of a glass melting furnace with a
power of 2 MW with oxygen.
The full furnace is supplied with oxygen by 10 similarly
dimensioned exchangers. The power of each of these exchangers is
adjusted to better distribute the total power necessary to operate
the furnace.
* * * * *
References